Apparatus and Method of Producing Net-Shaped Components from Alloy Sheets

ABSTRACT

A method and apparatus for producing ultra-fine grained metal material sheets. The apparatus molds and rapidly solidifies a metal material to form a fine grain precursor. The precursor is then subjected to a series of successive alternating tensile and compressive strains that alter the grain structure of the precursor so as to form a ultra fine grained structure in sheet form. The sheet form may then be subjected to superplastic forming to form a net shaped article.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No. 60/691,747 filed on Jun. 17, 2005.

BACKGROUND

1. Field of the Invention

The present invention relates to producing net shaped components of increased strength. More particularly, the invention relates to producing sheet components, having micrometer sized grain structures, that can be subsequently used in the production of net shaped sheet components of increased strength.

2. Related Technology

Over the last several decades, magnesium (Mg) alloy development has been inhibited by certain barriers. While wrought magnesium has the potential for making thinner structures, anisotropy in mechanical properties limits the applications of Mg alloys and their wrought products. The strength of Mg alloys is rather low in certain directions in comparison to most widely used structural materials, such as steels and precipitation-hardened aluminum (Al) alloys. For example, in-plane compression, yield strength can be only 85 MPa in basal textured Mg alloy; AZ31 B Mg alloy sheet in the H24 temper could have a high-value normal anisotropy parameter (R) of 3.2 in the transverse direction. A high value of normal anisotropy in sheet is helpful for deep drawing, but may not be suitable for other applications, particularly if in-plane strength is also anisotropic. In fact, this base element has not been a friendly host for extensive alloy strengthening.

The alloying elements that improve corrosion resistance and castability, such as Al, unfortunately introduce eutectic intermetallic phases. These envelope the primary grains in a coarse and brittle morphology. Furthermore, it is difficult to attain efficient age hardening by fine precipitates within the grains, as exemplified by the case of inefficient Al additions. Elements that promote age hardening, such as rare earth metals, are costly, detrimental to castability and ineffective in resisting corrosion. As a consequence of these barriers, increases in strength have been marginal, at best, and decade-old Mg alloys, such as AZ31 and AZ91D, still dominate the tonnage of commercial sheet and casting markets.

Hexagonal close packed (HCP) structured Mg alloys have low symmetry of slip systems that contributes to high anisotropy of mechanical properties. At room temperature, “basal a” slip {0001}

1120

is predominant, while “prism a” and

c+a

slip are difficult because of their significantly high critical resolved shear stresses (CRSS), which are reported in regions of high stress concentration such as grain boundaries and twin interfaces. Deformation twinning is often observed in polycrystalline Mg to compensate for the insufficiency of independent slip systems. The most common twinning modes are {1012} and {1011} twinning which accommodate the c-axis extension and contraction, respectively. If profusion of “tension” and “compression” twinning occur homogeneously, good strain hardening and large ductility could result in titanium (Ti) and zirconium (Zr). However, in Mg, twinning is inhomogeneous, and different modes of twinning are not initiated simultaneously. A single twinning mode can not fully accommodate plastic deformation. When basal slip is inhibited at ambient temperatures, twinning deformation can be localized, which leads to low ductility in Mg.

Two major drawbacks restrict application of wrought Mg alloys. First the symmetry of hexagonal close packed crystal structure has the effect of limiting the number of independent slip systems, thus providing alloys with poor formability and ductility near room temperature. Second, forming of Mg alloys at elevated temperatures (>300° C.), although helpful to overcome the restriction of slip, makes oxidation problems more severe.

Another means to strengthen Mg alloy, relative to Al and steel, is by grain refinement. By the well-established Hall-Petch relation, strength is proportional to d^(−1/2), where d˜grain size. Whereas conventional Mg alloy sheets and extrusions have a grain size in the range of 10 to 90 μm, reducing the grain size to about 1 μm or less (thus nanostructured and herein referred to as an “ultrafine grain size”) offers a striking opportunity to escalate the strength/density of Mg to levels above Al and steel. An ultrafine grain size could enable superplastic deformation to be carried out at lower temperatures and higher strain rates. At room temperature, grain refinement strengthens many polycrystalline metals. This is true for cubic structured metals such as Al, copper (Cu) and iron (Fe). However, for HCP metals, such as Mg alloys, grain refinement may also cause texture variation, and inadequate strengthening in certain directions.

Expensive and elaborate schemes to reach an ultrafine grain size in Mg have been developed in various research and development efforts. A number of known methods of grain refinement, such as rapid solidification, vapor deposition, and powder processing are practiced in the lab. These processes are costly, time consuming and have not enjoyed commercial success. Several other schemes for severe plastic deformation (SPD) have proven to be unpractical for manufacturing larger quantities of ultra-fine grain metals. Techniques now available for performing severe deformation of bulk material include reciprocating extrusion, three-axis plane-strain forging, torsion under hydrostatic pressure, and equal angular channel extrusion or pressing (ECAP). When these processes are used in a repeated manner, the overlap of shear zones within the bulk material from individual steps causes extensive grain subdivision and formation of fine grain structure. Concurrent recovery and recyrstallization processes transform subgrains with low-angle boundaries into high-angle boundary grains. It is generally accepted that grain refinement leads to a decrease in strain hardening as yield strength is increased, but the variation of strain rate sensitivity with grain refinement in Mg alloys is not clearly documented. This may offset any loss in ductility due to reduced strain hardening. The combination of strain hardening and strain rate sensitivity provides a synergistic effect of a higher tensile elongation, even when strain hardening exponent n (=d(log σ)/d(log ε)) may be lower. Deformation by several deformation passes through equal channels (so called ECAP) has been practiced in the lab on Mg bars, but is not practical for Mg sheets.

Since 1999, the University of Michigan conducted research aimed at producing sheets or billets of ultra-fine grain size, without using a closed shearing die as used in ECAP, but by using a multiple corrugation and flattening (MCF) process or a sine wave deformation process (SWP) that might be more suitable for a sheet product. The potential of the process was demonstrated with various aluminum alloys containing dispersoid particles. This repeated reversed plastic deformation approach was shown to achieve very fine grain size on the surface of the sheet, which progressively reaches the core regions of the sheet after several repeated passes. That research showed that changes in alloy chemistry, and use of dispersoid particles in the alloy, can take advantage of this simpler process to produce ultra-fine grain alloys. Application of this and other approaches to magnesium alloys was a subject of significant interest since these alloys possess inherently low ductility.

Under basic research funding from the National Science Foundation, it was previously demonstrated by the University of Michigan that, although hexagonal close packed metals, like Mg, have inherent problems with the breakdown of coarse grains due to textural and twinning-related issues, it is believed that either pure compression or a constrained SWP has the capacity to overcome these problems under suitable temperatures and process conditions.

Superplasticity is an attribute associated with fine-grained alloys. This plastic-type property is utilized commercially in automobiles and aircrafts to form complex net shapes in titanium (Ti) and Al. To date, Mg alloys have not enjoyed this advantageous processing in commerce. First, Mg alloy castings do not have the prerequisite grain boundary crystal structure and, secondly, wrought Mg sheets have been too coarse grained and/or too textured for superplastic forming.

Turning to more definitive discussion of nanotechnology, nano-size strengthening phases of about 100 nanometers are desirable within the grains. This is another strengthening mechanism, heretofore not available in weakly alloyed AZ31 sheets. However, construction and assembly of such a microstructure for bulk structural parts, ab-initio from nano-powders, is a very costly and laborious. Also, there are safety and health concerns for handling such fine particles in the workplace. It seems to be safer and more practical to generate such nano-strengthening particles in-situ during processing of the already assembled bulk component.

Grain size has a major effect on the formability of Mg alloy sheets. Currently, commercial wrought Mg alloy sheet is available only in low strength AZ31 alloy. It is fabricated from direct cast (DC) slabs (0.3 m thick) having a grain size of 200-1000 μm. Twin roll casting (TRC), a prototype process, is offered at 2 to 5 mm thicknesses with 60 to 2000 μm grain sizes and is currently only capable of 432 mm wide sheets. Fabrication from DC or TRC promotes strong texture because of the limited slip systems and twinning occurring in Mg alloys with such large grain sizes. Extrusions formed from such a base source are also textured to the extent that strength is 50% and toughness is 72% in one direction as compared to the cross direction. The grain boundary structure in conventionally prepared Mg alloy is not favorable to complex deformation without premature fracture, unless an elevated forming temperature is used. The pressing and deep drawing of 3-D shapes is limited by the texture and the inherent non-uniform deformation that results from twinning. Although twinning in some directions of the sheet causes increased elongation during tensile testing, twinning is an impediment to the formation of complex parts due to the anisotropy it produces in coarse grain Mg alloy, resulting in anomalies in work hardening and non-uniform deformation. Further, the modeling of forming processes and performance in the dies is not reliable with such non-uniformity in structure. Also, the coarse surface finish of present coarse grain Mg alloys poses a challenge to their acceptance as automotive sheet parts.

To minimize the adverse effects of coarse grains and twinning, conventional wrought alloy processes use multiple rolling and annealing operations until the grain size becomes finer. The TRC product is typically too thin to refine the grain size below 7 μm by such hot processing. The TRC structure also suffers from centerline porosity. Continuous cast Mg alloy may have promise, but currently this technology is not fully developed and many individual pieces of technologies are required for its full implementation, the scope of which is incompatible with small business operations and may not have the flexibility offered by the process of the present invention. Further, the slag and dross of known processes would be conducive to the attacking of the refractories by the processed material; SF₆ gas (a global warming gas) may be a manufacturing by-product; and trapped inclusions may result from any necessary flux. The many stages involved in breaking down large-grained conventional sheet precursors to produce the sheet form cause current wrought Mg alloys to be expensive, on the order of $5.00 to $10.00/lb.

As seen from the above, there exists a need for an apparatus and process that can be carried out in a rapid and automated manner so as to change alloy composition and grain structure, thereby allowing such processed alloys to be subsequently worked into net-shaped sheet products.

SUMMARY

In achieving the above object, the inventors have discovered a practical new process and apparatus to generate inexpensive ultra fine grain structured sheets, where grain sizes of less than or equal to about 2 μm are achieved, which can be subsequently deformed via superplastic forming processes to form net shaped, sheet formed articles. Various metals and alloys can be employed with the present invention, including, but not limited to, Mg, Al, zinc (Zn), nickel (Ni), copper (Cu), α/β Ti, steels, duplex α/γ stainless steels, α/γ steels, γ/martensite Maraging steels and metal/ceramic particle composites.

The present process involves the sine-wave deformation processing (SWP) of fine grain structured sheets initially formed from various rapid solidification molding methods that can produce an ultra fine grain precursor, including injection molding and variations on injection molding, extrusion molding and twin roll casting. Thereafter, the final net shaping of parts can be accomplished by superplastic forming, drawing or stamping, etc. Thus, the present invention provides for the initial formation of a fine grained precursor having a grain size of less than about 10 μm. Thereafter, the fine grained precursor is subjected to SWP, which breaks down the microstructure of the precursor and produces new grain boundaries. The resulting sheet has an ultra fine grain structure lending itself to final net shaping by superplastic forming processes. Accordingly, in one aspect the present invention is a method of forming a sheet material having a refined grained structure, the method comprising the steps of: providing a metal material; molding and rapidly solidifying the metal alloy to form a fine grain precursor; and imparting plastic deformation to the fine grain precursor by a combination of alternating tensile strain and compressive strain to form an ultra fine grain structured sheet form.

In another aspect, the step of molding and solidifying develops a mutliphased microstructure in the fine grained precursor.

In a further aspect, the multiphased microstructure includes pinning particles that minimize grain growth.

In yet another aspect, the step of imparting plastic deformation includes the step of storing dislocations in the microstructure.

It is also an aspect that step of imparting plastic deformation includes the step of causing the formation of new grain boundaries having high misorientation suitable for warm forming or superplastic forming.

Still another aspect is that the molding step and the imparting plastic deformation step are performed in an integrated apparatus.

A further aspect is that the molding step and the imparting plastic deformation step are performed by separate machines.

In another aspect, the molding step includes semisolid metal injection molding of the metal material.

In a further aspect, the molding step includes one of extruding of the metal material and twin roll casting of the metal material.

In yet another aspect, the imparting plastic deformation step includes corrugating the precursor in a first direction and subsequently corrugating the precursor in a second direction.

It is also an aspect the second direction is orthogonal to the first direction.

Still another aspect is that the second direction is aligned with the first direction.

A further aspect is that the imparting plastic deformation step further includes the step of flattening the precursor.

In another aspect, the flattening step is performed after at least one of the steps of corrugating the precursor in the first direction and the second direction.

In a further aspect, the imparting plastic deformation step further includes the step of corrugating in a third direction and a fourth direction.

In yet another aspect, a second flattening step is performed after at least one of the third and fourth corrugating steps.

In an additional aspect, after the step of imparting plastic deformation, the step of net shaping the nano-sized grain structure sheet.

In another aspect, the invention further includes the step of heat treating the net shaped part to impart creep resistance to the net shaped part.

In still another aspect, the step of net shaping includes one of stamping, drawing, deep drawing and superplastic forming.

In another aspect, the step of net shaping forms an automotive component.

In another aspect, the invention includes an apparatus for performing the above mentioned method.

In yet another aspect, the invention includes an article formed by the above mentioned method.

In another aspect, the invention further includes the step of providing the sheet form with a thickness being less than that of the precursor.

In another aspect, the metal material is a metal alloy.

In an additional aspect, the metal material is a magnesium alloy.

In another aspect, the metal material is one selected from the group of aluminum alloy, zinc alloy, nickel alloys, copper alloy, α/β titanium alloy, steels, duplex α/γ stainless steels, α/γ steels, γ/martensite Maraging steels and metal/ceramic particle composites.

In further another aspect, the step of imparting plastic deformation includes die pressing of the fine grain precursor.

In another aspect, the step of imparting plastic deformation includes rolling the fine grain precursor.

In another aspect, the sheet form is provided having a grain structure of less than about 2 micrometers.

In still another aspect, the sheet form is provided having a grain structure of less than about 1 micrometer.

In an additional aspect, the step of imparting plastic deformation is performed while the precursor is heated above ambient.

In one aspect, the step of imparting plastic deformation imparts tensile strain and compressive strain in a strain direction.

In another aspect, the step of imparting plastic deformation is performed by passing the precursor in a first direction between at least one pair of deforming members having corrugated surfaces, and wherein the first direction is orthogonal to the strain direction.

In yet anther aspect, the compressive strain is imparted at least in part by flattening the work piece while constraining lengthening of the work piece in the strain direction.

In still another aspect, constraining lengthening of the work piece in the strain direction is done so as to achieve one of decreasing, increasing or preserving the thickness of the precursor in the sheet form

In another aspect, the invention is an apparatus for refining grain structure and producing ultra-fine grained metal material sheets, the apparatus comprising: a receptacle having an inlet, a discharge outlet remote from the inlet, and a chamber defined between the inlet and the discharge outlet; a feeder coupled with the inlet, the feeder configured to introduce an metal material into the chamber via the inlet; a heating device for transferring heat to the metal material located within the chamber such that the metal material is at a temperature above its solidus temperature; discharge means for discharging the metal material from the receptacle through the discharge outlet; forming means for forming and rapidly solidifying the discharged metal material into a fine grained precursor; plastic deformation means for imparting strain into the precursor article and deforming the precursor article into a corrugated work piece, the plastic deformation means including a pair of opposing forming members having protrusions formed on a surface thereof, the protrusions of one forming member being offset from the protrusions of the opposing forming member; and flattening means for flattening the corrugated work piece into a sheet form of the metal material having an ultra-fine grain size.

In another aspect, the opposed forming members are pressing dies or rolls.

In further another aspect, the plastic deformation means includes means for imparting tensile strain and compressive strain into the precursor article.

In another aspect, the plastic deformation means includes means for imparting first corrugations oriented in a first direction into the precursor article and subsequently imparting second corrugations oriented in a second direction.

In another aspect, the second direction is orthogonal to the first direction.

In still another aspect, the second direction is aligned with the first direction.

In another aspect, the plastic deformation means is configured to impart third corrugations and fourth corrugations into the work piece.

In an additional aspect, the third and fourth corrugations are respectively oriented in the direction of the first and second corrugations.

In yet another aspect, the third and fourth corrugations are out of phase with the first and second corrugations.

In another aspect, the third and fourth corrugations are respectively 180 degrees out of phase with the first and second corrugations.

In another aspect, the invention further includes net shaping means for shaping the sheet form of the metal material into a net-shaped article.

In further aspect, the shaping means is one of a drawing press and a superplastic forming machine.

In another aspect, the receptacle, feeder, heating means, discharge means and forming means are part of an injection molding machine.

In an additional aspect, the receptacle, feeder, heating means, discharge means and forming means are part of a semi-solid metal injection molding machine.

In another aspect, the invention is a method of forming a sheet material having a refined grained structure, the method comprising the steps of: providing a metal material; molding and rapidly solidifying the metal alloy to form a fine grain precursor defining a line length; initially increasing the line length of the precursor to form a work piece; decreasing the line length of the work piece and then increasing the line length of the work piece; and flattening the work piece to form an ultra fine grain structured sheet form.

In still another aspect, the step of flattening the work piece is performed before the decreasing and increasing step and then again after the decreasing and increasing step.

In another aspect, the step of flattening the work piece is performed after the decreasing and increasing step.

In an additional aspect, the initially increasing step introduces strain into the work piece in a first direction.

In further aspect, the decreasing and increasing step introduces strain into the work piece in a second direction.

In another aspect, the second direction is orthogonal to the first direction.

In yet another aspect, the second direction is generally aligned with the first direction.

It is also an aspect of the invention that the plastic deformation means imparts tensile and compressive strain in a strain direction that is orthogonal to a direction through which the work piece is passed through the plastic deformation means.

It is a further aspect that the flattening means imparts, at least in part, compressive strain to the work piece.

It is yet another aspect that the flattening means includes features to control lengthening of the workpiece in the strain direction whereby the thickness of the sheet form may be controlled so as to be increased, decreased or the same as the thickness of the precursor.

Further features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic illustration of a manufacturing cell embodying the principles of the present invention;

FIG. 2A is a perspective view of the longitudinal roll dies shown in FIG. 1 and which may be utilized with the present invention;

FIG. 2B is a perspective view of the transverse roll dies seen in FIG. 1 which may be used with the present invention;

FIG. 2C is perspective view of flattening roll dies as seen in FIG. 1 and used in connection with the present invention;

FIG. 2D is perspective view of a pair of pressing dies that may be utilized as an alternative to roll dies in accordance another embodiment of the present invention;

FIG. 3 is a flowchart of one possible process in accordance with the present invention;

FIG. 4 is a flowchart of another possible process incorporating the principles of the present invention;

FIG. 5 is a diagrammatic illustration of the present invention incorporating an extrusion device;

FIG. 6 is a schematic illustration of a twin roll casting device incorporated with the present invention;

FIG. 7 is a graphical comparison of the effect of grain size (d) on hardness (Hr) for SWP AZ91D and AZ31B;

FIG. 8 is a representation of the results of a superplastic bulge test (processed at 280° C. and 200 psi) as a function of initial grain size; and

FIGS. 9A, 9B, 9C and 9D are schematic illustrations of a precursor undergoing SWP according to another embodiment of the the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to one aspect and embodiment of the present invention, a fine grained precursor is formed by the injection molding of metal, such as by the Thixomolding™ process of Thixomat, Inc., Ann Arbor, Mich. According to this process, melt temperatures can be lowered to near liquidus, some 80 to 100° C. lower than in DC or TRC. These lower temperatures assist in faster cooling to nucleate finer grains upon solidification. As injection molded, Thixomolded™ Mg alloys are isotropic with 4 to 5 μm grain size a phase. Through the use of multiple feeding ports, the rapid injection molding of large sheet bars is possible. Suitable sheet bar would be readily molded in existing commercial Thixomolding machines, of sizes up to 1000 tons, with sheet dimensions of 20×400×400 mm.

Table 1 presents various production methods for a precursor work piece, such as sheet bar, as well as for a range of grain sizes resulting from that production method including the method of the present invention. TABLE 1 Effect of Process on Grain Size of Mg Sheet Process Condition Grain Size (μm) Direct Cast As Cast Billet (300 mm) 200 Direct Cast Extruded 8-90 Twin-roll Cast As Cast (2-5 mm)  60-2000 Twin-roll Cast Hot Rolled 7-10 Injection Molded As Molded 4-5  Injection Molded + SWP As SWP ≦1

Referring to FIG. 1, this figure schematically illustrates an apparatus, generally designated at 8, embodying the principles of the present invention. The apparatus 8 includes a molding machine 10 for the metal injection molding of sheet bar. As seen in FIG. 1, the construction of the molding machine 10 is, in some respects, similar to that of a plastic injection molding machine. The machine 10 is fed with feedstock 11 via a hopper 12 into a heated, reciprocating screw injection system 14 which maintains the feedstock under a protective atmosphere such as argon. More particularly, the feedstock is received into a barrel 15 via an inlet 16 located at one end of the barrel 15. Within the barrel 15, the feedstock is moved forward by the rotating motion of a screw 18. As the feedstock is moved forward by the screw 18, it is also heated by heaters 20 (which may be a resistance, induction or other type of heater) while being stirred and sheared by the action of the screw 18. This heating and shearing is done to bring the feedstock material into a injectable state. This injectable material passes through a non-return valve 22 and into an accumulation zone 24, located within the barrel 15 beyond the forward end of the screw 18. Upon accumulation of the needed amount of injectable material in the accumulation zone 24, the injection portion of the cycle is initiated by advancing the screw 18 with a hydraulic or other actuator 25. Advancement of the screw 18 causes the material in the accumulation chamber 24 to be ejected through a nozzle 26 into a mold 28 filling the mold cavity defined thereby and forming a precursor work piece such as sheet bar 30. This initial formation of the precursor allows developing of a multiphase microstructure with pinning particles or phases to pin grain boundaries to minimize grain growth.

In one preferred embodiment, the metallurgical process of the machine 10 results in the processing of the particulate feedstock into a solid plus liquid phase prior to its injection into the mold 28. Various versions of this basic process are known and two such versions are disclosed in U.S. Pat. Nos. 4,694,881 and 4,694,882, which are herein incorporated by reference. The process generally involves the shearing of the semisolid metal so as to inhibit the growth of dendritic solids and to produce non-dendritic solids within a slurry having improved molding characteristics which result, in part, from its thixotropic properties. (A semisolid non-dendritic material exhibits a viscosity that is proportional to the applied shear rate and which is lower than that of the same alloy when in a dendritic state). Variations on this process of forming the sheet bar 30 may include providing the alloy material initially in a form other than a particulate; heating the alloy material to an all liquid phase and subsequently cooling into the solid plus liquid phase; utilize twin screws for processing the alloy; employing separate vessels for processing of the alloy and injecting of the alloy; utilizing gravity or other mechanisms to advance the alloy through the barrel to the accumulation zone; alternate feeding mechanism, including electromagnetic; and other variations on the process.

In another preferred embodiment, metallurgical process of the machine 10 results in the processing of the particulate feedstock into an all liquid phase that is injected into the mold 28 and rapidly solidified.

Once the fine grained sheet bar 30 is formed, it is subjected to SWP. Generally, SWP involves the imparting of plastic deformation by a combination of alternating tensile and compressive strains or deformations. This second step permits storage of dislocations within the microstructure, which leads to the formation of new grain boundaries with high misorientation suitable for subsequent warm forming or superplastic forming.

In one implementation of the SWP process, the precursor is subjected repeated shaping of the material between a pair of corresponding members having corrugated or sine-wave shaped forming surfaces. The shape of the forming surfaces impart large strain, breakdown the cast microstructure and produce new grain boundaries in the precursor. Beginning with sheet bar 30, initially formed so as to have a fine grain structure of 10 μm or less, this precursor work piece is then shaped, with or without lateral constraint, between two members having corresponding corrugated forming surfaces, in what is essentially a plane-strain stretch-bend operation. After the first shaping, the work piece is again shaped, During this second shaping, however, the corrugations are preferably, but not necessarity, oriented in a direction different from the corrugations of the first shaping. An orthogonal orientation for the second shaping is believed to produce the best end results. Preferably, the two shaping steps are then repeated with the corrugations in these third and fourth steps being the inverse of those seen in the first two shaping steps. By the term inverse, what is meant is that the ridges and valleys of the third corrugation are reversed or out of phase from the ridges and grooves of the first corrugations. Thus, these subsequent shaping cause a reverse deformation (pushing in the opposite direction) of ridges resulting after the first two shapings. After all four shaping steps, and additional shaping steps if desired, the work piece is preferably flattened to remove any waviness to the shape. As further discussed below, the work piece may alternatively be flattened between each shaping step or after the first two shaping steps.

The alternating tensile and compressive deformations are imparted by creating a general sine wave shape in the precursor. This shaping increases the line length (the length of the centerline of the precursor). Following this with a reverse sine wave shaping of the work piece results in the line length initially being shortened and then again lengthened as the raised ridges of the shape are converted in recessed valleys. When undergoing flattening, the line length is thus shortened. Accordingly, increasing the line length introduces tensile strain into the work piece and reducing the line length introduces compressive strain.

Preferrably, SWP is conducted at a warm temperature and the deformation temperature of the material is progressively lowered after each pass, for example, starting at 250° C. and decreasing to 170° C. for the final flattening step. This can be achieved in several ways, including providing heated shaping members or rolls, as described below.

After four alternate shaping steps, the strain in the mid-plan has gone from tension to compression to tension again in both the 0° and 90° directions. With these successive shapings, all parts of the sheet bar material are deformed in a manner of “kneading”, by incorporating reversed plane strain bending and plane strain stretching in two orthogonal directions of the plate. The repeated deformation by the process causes accumulation of large plastic strain and breaks up the original grain structure within the work piece. Grain refinement occurs initially heterogeneously, but eventually homogeneously, over a large area. While four shapings are discussed in one of the preferred embodiments, more or less shapings may be possible to achieve the desired results.

Various schemes can be envisioned for deforming the sheet bar 30 during SWP. SWP can be achieved by passing the sheet bar 30 successively through a series of rolls or by pressing the sheet bar successively between a pair of opposing pressing dies, either of which may be heated. Also, SWP may be performed separately (at a remote location) from the formation of the sheet bar 30 or may be integrated directly into a processing cell whereby the apparatus 8 is provided with a transfer mechanism (which may be any known variety and which is represented by line 29) to transfer the sheet bar 30 from the mold 28 to a rolling or pressing mill 31. As seen in FIG. 1, the apparatus 8 includes a rolling mill 31, having a series of roll sets, integrated with the molding machine 10. The rolls of the roll sets may be impressed toward each other by backup rolls 33 (shown in phantom) as is commonly known.

In the illustrated rolling mill 31, the sheet bar is passed through a first set 32 of opposed corrugated rolls 34. The surfaces of the rolls 34 are each provided with corrugations 36 extending circumferentially about the rolls 34. The corrugations 36 of each rolls 34 generally correspond with respect to one another such that a ridge on one of the rolls 34 is received in a valley of the opposing rolls 34. As the sheet bar 30 passes through the first set 32 of rolls 34 a lengthwise corrugation, parallel to the direction of travel of the sheet bar 30, is imparted into the sheet bar 30. This results in a sine wave shape being imparted to the work piece that is oriented in a direction orthogonal to the direction in which the work piece is passed through the rolling mill 31. Accordingly, the induced strains, tensile and thereafter compressive, will be generally in the direction of the sine wave shape itself. In order to constrain lateral expansion of the sheet bar 30, one of the rolls, the lower roll 34 in FIG. 1, may be provided with raised lands 38 on the opposing ends of the roll 34. The lateral most corrugations of the upper roll 34 fit within and extend below the uppermost surface of the lands 38. As a result, the sheet bar 30 is constrained from expanding laterally beyond the lands 38 of the rolls 34. These rollers, 34 are separately illustrated in FIG. 2A apart from the subsequent rollers. Alternately, lateral expansion may be unconstrained.

Having been corrugated or worked by the first set 32 of rolls 34, the worked sheet bar or work piece is passed to a second set 40 of rolls 42. Upon encountering this second set 40 of rolls 42, the work piece encounters corrugations 44 that are oriented orthogonally, 90 degrees from the corrugations 36 of the first set 32 of rolls 34. As such, the corrugations 44 are oriented axially with respect to the rolls 42 and transverse with regard to the direction of travel of the sheet bar 30. As with the prior set 32 of rolls 34, the corrugations 44 of the second set 40 of rolls 42 are provided such that the ridge of a corrugation on the upper roll 42 is received within the valley of a corrugation 44 of the lower roll 42. Raised lands 46 may be formed on the lower roll 42 so as to define constraints and prevent lateral lengthening/expansion of the work piece as it is passed through the second set 40 of rolls 42. The rollers 42 of the second set 40 are separately illustrated in FIG. 2B and could alternatively be provided such that lateral expansion is unconstrained.

From the second set 40 of rollers 42, the worked sheet bar is passed in the illustrated rolling mill 31 between a third set 48 of rolls 50 designed to flatten the worked sheet bar. To achieve this, the rolls 50 are provided with smooth surfaces 52 that engage and compress the worked sheet bar as it passes between the rolls 50. As the work piece is flattened, compressive strain in imparted to the work piece. Similar to the prior two sets 32, 42 of rolls 34, 42, the lower roll 50 of the third set 48 of rollers includes raised lands 54 to constrain and inhibit lateral lengthening/expansion of the worked sheet bar as it passes between the rolls 50. By adjusting the lateral position and constraint provided by the lands 54 of the rolls 50, the thickness of the resulting sheetstock material 78 can be controlled so as to be decreased, increased or the same as the original thickness of the sheet bar 30, all the while continuing to accumulate plastic deformation to the sheetstock material 78. The rolls 50 of the third set 48 are separately illustrated in FIG. 2C.

Once exiting the third set 48 of rolls 50, the work piece may again be subjected to corrugation and the process of passing the work piece through the three sets of rollers is repeated, as suggested by dashed line 56.

As an alternative scheme for deforming the sheet bar 30, pressing plates 58 (one set 59 of which is representatively illustrated in FIG. 2D) may be used in place of the sets of rolls 32, 40, 48. As seen in FIG. 2D, the plates 58 are provided with cooperating corrugations 60 in which the ridge of one corrugation interfits with the valley of the opposing corrugation. Similarly, a raised perimeter or land 62 is provided about the periphery of one of the plates, herein the lower plate 60, so as to laterally constrain the sheet bar 30 as it is pressed therebetween.

As thus far described, SWP occurs generally according to the process illustrated by the flowchart of FIG. 3. As shown therein, SWP starts at box 66 wherein a sheet bar 30 is received and subjected to corrugating in a lengthwise or parallel direction in box 68. After lengthwise corrugating of the sheet bar 30, the work piece undergoes transverse corrugation in box 70 and subsequently is flattened as indicated in box 72. After being flattened in box 72, the lengthwise and transverse corrugating of the work piece may be repeated as indicated by line 74. Optionally, as indicated by phantom line 76, the work piece can undergo subsequent lengthwise and transverse corrugation prior to being flattened in box 72. However, it is believed to be preferable that flattening according to box 72 occurs prior to subsequent corrugation of the work piece. After proceeding through the corrugation process wherein both lengthwise and transverse corrugation occurs twice (thus corrugating of the work piece four times) the work piece is finally flattened in box 72 and flat sheetstock material 78 is outputted and the process ends in box 80. While not illustrated, it is contemplated that the transverse corrugation of box 70 can be replaced with an additional lengthwise corrugation, aligned with or off set from the initial lengthwise corrugation.

While the above process has been described whereby a lengthwise corrugation of the sheet bar occurs prior to a transverse corrugation, it should be apparent that the two sets 32, 40 of rolls 34, 42 can be alternated in their positions such that the transverse corrugations 44 of rolls 42 first encounter the sheet bar 30 and the lengthwise corrugations 36 of rolls 34 encounter the work piece after initial corrugation. This order of the rolls may be advantageous in that the orientation of the corrugations 44 would allow for a “grabbing” of the sheet bar 30 possibly facilitating entrance of the sheet bar 30 into the initial rolls, as well as subsequent rolls.

Two alternative methods for SWP are illustrated by the flowchart of FIG. 4. According to this process, after each corrugation of the work piece, a flattening of the work piece occurs. Accordingly, SWP begins in box 82 where sheet bar 30 is transferred to the first set 32 of rolls 34 where it undergoes lengthwise corrugation in box 84. After lengthwise corrugation, an additional set of rolls, similar to the flat rolls 50 of the previously mentioned third set 48 are provided so as to flatten the work piece in box 86. The worked and flattened work piece is thereafter transferred to another set of rolls where transverse corrugation occurs in box 88. After transverse corrugation of the work piece, a set of rolls again flatten the work piece in box 90. At this stage the work piece is transferred back to the first set of rollers, according to line 92, where lengthwise corrugation is again performed in box 84. The work piece then proceeds through flattening (box 86), transverse corrugation (box 88), flattening (box 90) and the formed flat sheetstock material 78 is produced at the end of SWP as indicated by box 94.

As mentioned above, the alternating tensile and compressive deformations are imparted by creating a generally sine wave shape in the precursor 30. FIGS. 9A-9D illustrate the concept of line lengthening and shortening, as well as lateral constraint, in another embodiment of the invention.

As seen in FIG. 9A, the precursor 30 undergoes plastic deformation in a set of non-constraining rolls 334 that are generally analogous to the rolls 34 of FIG. 1. The rolls 334 include mating or corresponding corrugations 336 that extend circumferentially around the rolls 334. As a result, a generally sine wave shape is imparted into the worked precursor (the work piece), generally orthogonally to the direction of rolling. In that the rolls 334 are non-constraining, the work piece is lengthened along its centerline visa vi the sine wave shape. An initial rolling or corrugating of the precursor 30 thus lengthens and imparts tensile strain to the work piece. Subsequently, as seen in FIG. 9B, the work piece is at least partially flattened (imparting compressive strain) by the flat surfaces 352 of rolls 350, which are generally analogous to the rolls 50 of FIG. 1. Rolls 350 laterally constrain, via lands 354, the work piece as it is flattened and as such the line length is reduced during flattening. Next, as seen in FIG. 9C, the work piece under goes a corrugation between another set of rolls 360 having corrugations 362 that are circumferential or parallel to the rolling direction of the rolls 360. This corrugation is a reverse corrugation, however, in that the corrugations 362 of this set of rolls 360 are reversed from corrugations of corrugated roll 334. In other words, the peaks and valleys of these rolls are generally oppositely oriented relative to those of the prior corrugated rolls 332. If the work piece has not been completely flattened by flat rolls 350, this set of corrugated rolls 362 will first compress and then stretch the work piece as previously described elsewhere in this specification, resulting in the overall lengthening of the line length of the work piece. Again, the corrugation may be done without lateral constraint by the rolls 362. Finally, as seen in FIG. 9D, the work piece is compressed and the sine wave shape completely flatten between a set of rolls 372. (Obviously, intermediate rolls 362 and the final set of rolls 372, the work piece may undergo any number of corrugations cycles (where the strain is aligned, orthogonal to or otherwise oriented with respect to the previously induced strain), or other processing steps, imparting tensile and compressive strains so as to accumulate deformation in the work piece.) At rolls 372, the work piece is laterally constrained by lands 374 so that the final sheet form 378 is outputted for further processing. By controlling the lateral contraint of the workpiece, it is possible to produce a resultant sheet form that has a thickness that is decreased, increased or the same as the thickness of the precursor.

It is noted that the rolls of FIGS. 9A-9D are illustrated in a side by side positioning. As such, the construction is representative of a reverse rolling mill. In such a reverse rolling mill, the work piece is provided through one roll in one longitudinal direction and through a subsequent roll in a generally reversed longitudinal direction. Reverse rolling mills themselves are well known in the art and further elaboration on the construction is therefore warranted herein.

As previously mentioned, various schemes for the manufacturing of the initial precursor, the sheet bar 30 discussed above, are believed possible if proper and precise control of the manufacturing process and rapid solidification thereof is done. FIGS. 5 and 6 schematically illustrate two additional manufacturing schemes wherein the injection molding machine 10 of the first embodiment is alternately replaced with an extrusion machine 110 (in FIG. 5) and a twin roll casting machine 210 (in FIG. 6).

Referring now to FIG. 5, the extrusion machine 110 includes a barrel 112 within which is located a screw 114. In that the other components of an extrusion machine are well known to those skilled in the art, additional discussion of the extrusion machine 110 is not provided herein. Material is extruded from the extrusion machine 110 and rapidly solidified between a pair of molds 116 such that a continuous sheet of solid material is transferred from the extrusion machine to the rolling mill 31. By precisely controlling the process of the extrusion machine, it is believed that the required fine grained microstructure can be achieved in a continuous sheet, which operates as the precursor material into the rolling mill 31 in accordance with the present invention. The rolling mill 31 illustrated in FIG. 5 is similar to the rolling mill 31 discussed in connection with the prior embodiment. Accordingly, reference is hereby made thereto and further discussion is not required.

As seen in FIG. 6, the twin roll casting machine 210 includes a pair of counter-rotating rolls 212 which receive the processed material 214 from a processing vessel 216. Between the rolls 212, the material 214 is rapidly solidified into a precursor sheet form 218 of a first thickness 220. By precisely controlling the twin roll casting machine 210, it is anticipated that the fine grain microstructure required according to the present invention can be achieved. Accordingly, by transferring the precursor material 214 to a rolling mill 231 similar to that previously described, the material 214 can be reduced to a final sheet form 222 having the desired reduced thickness 224. Since the rolling mill 31 of FIG. 6 is substantially of the same construction as that previously described in connection with FIG. 1, reference thereto is herein made.

With a 400×400×20 mm sheet bar 30 as the precursor, the above described SWP process can reduce the thickness of the sheet to about 2 mm, wherein the final sheet dimensions could be 1250×1250 mm. With thinner starting materials, such as 6.35 mm hot rolled plate, it is anticipated that the thickness can be reduced to about 1 mm. Alternatively, the SWP process can produce a sheet maintain the original starting thickness of the precursor or can actually produce a thickened sheet. The latter is achieved by further constraining lateral expansion of work piece, after the work piece has been shortened via a corrugating step, to a dimension that is less than the starting dimension of the precursor.

When an integrated automated manufacturing cell, such as one of those previously described, combines the rapid solidification of metal injection molding with SWP as part of the same manufacturing cycle, the rate of production in one machine is anticipated to be about 1 sheet bar per 20 seconds.

As would be surmised from the preceding discussion of the invention, the as-molded grain size and a content of an injection molded metal sheet bar is a favorable starting point to attaining sub-micron grain size and low-anisotropy in the subsequently SWP sheet. It appears that SWP, with its vigorous thermomechanical working, subdivides intermetallic particles into nano-sizes, and, probably, encourages partial solution and more homogeneous reprecipitation of fine arrays within the grains. Some sub-divided residual β phase could serve to pin grain boundaries during dynamic recrystallization and heat treatment. The subdividing of this inherently coarse β phase is beneficial to the ductility of Mg alloys. The same role is anticipated as being possible with two phase alloys, such as α/β Ti, α/γ stainless steels, γ/martensite Maraging steels and cementite steels.

The aforementioned β phase effect is but one aspect of the new opportunities to redesign Mg for this new process. The literature is replete with new Mg alloying discoveries that have yet to be applied to a low cost sheet form. These alloying additions are easily reduced to sheet form by the present invention, especially utilizing “blending” techniques. Such alloying additions as Ca, Sr, Y, Zr and Zn—Y can boost the modest strength of the commercial sheet alloy AZ31. Additionally, the large melts and alloy cross contamination, which are inherent in DC and TRC, can be avoided by using the above mentioned injection molding SWP process. Purging of the previous alloy and addition of granules of new blends can be accomplished in minutes in an injection molding machine, without the wasted crucible charges, slag and dross typically associated with DC or TRC operations.

Ductility during warm temperature stamping (and superplastic forming) of metals is enhanced by the presence of many grain boundaries, but grain boundaries developed from current casting processes are unsuitable for forming applications because they do not permit rolling or sliding between grains. Grain boundary character has a major effect on the phenomena of sliding and shearing properties of grain boundaries during deformation. Even at modestly elevated temperatures (150-200° C.), Mg alloys can be formed easily by warm forming processes, provided they have a fine grain structure (about 1-3 μm) and favorable grain boundaries produced by deformation processing. While forming of an alloy at room temperature is preferred, 150-200° C. temperatures are not unusual for inexpensive forming applications (plastics are often formed at such temperatures). Unlike plastics however, Mg parts can be heat treated to grow larger grain size and become creep resistant, or can be alloyed appropriately to make them creep resistant. Low temperature forming can however keep energy usage low during forming and avoid undesirable oxidation encountered during the superplastic forming process.

The rapid solidification during the injection molding process provides a fine grain structure that does not exhibit twinning during subsequent deformation. However, grain boundaries created from the liquid state are crystallographically related, and may possess “special” boundaries that do not permit grain boundary sliding. Special boundaries may have high misorientation angles, but they could have a significant fraction of coincident lattice sites (CSL) and low grain boundary energies to make sliding difficult. While the strain contributed by grain boundary sliding is not large during warm forming, if it is capable of providing accommodation locally, it prevents fracture of the material along grain boundaries. Thus, the boundaries required for enhanced formability must not be those produced by the casting process, but those generated by the plastic working process. The plastic working generates additional dislocations near the grain boundaries and renders then into configurations of higher disorder or higher energy, suitable for enhanced formability.

The above understanding, although derived from basic principles of materials science, has been repeatedly verified in practice with costly consolidated (rapidly solidified powder) RSP alloys. Extensive deformation of injection molded material and the like, to change grain boundary character, requires the special kneading process that is accomplished in the present approach with SWP. The other approaches available for such extensive deformation (e.g. ECAP, high pressure torsion), do not appear suitable for commercial scale-up, nor can they be easily automated for producing thin, wide sheets.

Accordingly, via the present invention, an end resultant can be produced, by initially providing a net-shape sheet bar alloy with a uniform microstructure and an original fine grain size of less than 10 μm through rapid cooling during forming, without segregation through the thickness of the material. This can be achieved by various forming methods including injection molding and other variations on injection molding, including semi-solid metal injection molding, extrusion molding, TRC (hot rolled). Afterward, the microstructure is refined to a nano-structure by processing the sheet into an untextured sheet that exhibits superior formability. This can be achieved by hot-pressing, rolling or other processes utilizing appropriately shaped surfaces in the dies as previously discussed. The final net-shaped part is thereby after formed by either superplastic forming (SPF), warm drawing, warm stamping or other methods. (Initial grain size may be reduced to lower the SPF working stress, to lower the SPF temperature for better surface finishes, and to raise the SPF rate.) One the net-shaped part is formed, optional heat treating (annealing, etc.) may be done to the final part to grow the grains so as to stop SPF and to impart creep resistance to the final article. As a result, what is attained is an inexpensive, light-weight part with very high strength to weight ratio, along with enhanced toughness.

As seen above, the process starts with un-textured sheet alloy having a fine grain size of less than 10 μm. However, the sheet alloy may be two phase and/or include high-angle grain boundaries; the former to control grain growth, promote grain boundary shear during SPF and strengthen the final part, and the latter to promote final net shaping and decrease texture. In refining the microstructure to obtain a grain size of about 1 micron, severe slip deformation is imparted to generate simultaneous recrystallization to micron-sized grains faced with high-angle grain boundaries. Thereafter, the coarse second phases are further sub-divided and/or reprecipitated into nano-sized arrays. In the above, twinning and the generation of textures are both minimized.

As an example, a commercial AZ31B Mg alloy in the form of hot-rolled plate, with thickness of 6.35 mm, was used as a precursor material work piece. The chemical composition of this alloy is 3.0 wt % Al, 1.0 wt % Zn, 0.45 wt % Mn and the balance Mg. An 89×89 mm square work piece was cut from the as-received plate, and then processed by SWP as described above. The initial bimodal structure of the as-received alloy was refined into a nearly uniform ultrafine grain structure. The bimodality of the initial structure and its change toward a more uniform structure were characterized by a detailed grain size distribution analysis using known computer image analysis software. Based on image analysis, the initial bimodal microstructure of the as-received alloy contains 31% area fraction of coarse grains of size 22.1 μm, but has an average grain size of 9.8 μm. The final microstructure after SWP had an average grain size of 1.4 μm, which contained less than 3% area fraction of coarse grains.

Mechanical properties of AZ31B Mg alloy for different alloy processing conditions at room temperature are presented in Table 2 in terms of strength, elongations (including uniform and post-uniform elongations), and normal anisotropy ratio (R). In comparing tensile stress vs. strain for coarse grain as-received and ultrafine grain as-processed alloy, it is seen that both strength and ductility are greatly improved due to grain refinement. The yield strength increases from 160 MPa to 280 MPa, and the tensile elongation increases from 13.0% to 22.6%. The presence of very coarse grain bands in the surface of the as-received alloy appears not to change the yield strength, but deteriorates the alloy's ductility. The post uniform strain depends on strain rate sensitivity, m. The variation of m for the as-received, as processed and annealed materials in terms of grain size, and the corresponding uniform strains, show that the m value decreases with increasing the grain size. The m value of the as-processed alloy is found to be more than four times that of the as-received alloy. This is a remarkable improvement at room temperature. Uniform strain does not change with the grain size monotonically. The ultrafine grained sample has the lowest uniform strain. With grain size increasing, the uniform strain first rises, and then decreases after a critical size of about 5 μm.

Anisotropy in strength and ductility was also examined. For in-plane compression, the as-received alloy yielded at a stress of 60 MPa, which is rather low. After a low rate of strain hardening up to a strain of 0.05, strain hardening rate increased remarkably with increasing strain (at this point the hardening curve is concave). For in-plane tension and normal-to-plane compression, a much higher yield stress and a lower rate of strain hardening were observed. In addition, for normal-to-plane compression, the rapid strain hardening and high yield strength caused failure of tested specimen at a low strain (ε=0.06). For in-plane compression of the as-processed alloy, the yield strength was lower than that for tension, but a significant increase in strength occurred in comparison with the as-received alloy, and no concave curve for strain hardening behavior is observed. The difference in the yield strength for in-plane tension and compression decreased as grain size becomes finer. Similar behavior is seen in terms of strain hardening. This suggests that the anisotropy in strength for in-plane deformation decreases due to grain refinement. In normal-to-plane compression of both, the as-processed and as-received materials have similar yield strength, but the ultimate tensile strength for the fine grain processed material is higher indicating that strain localization in the coarse grain alloy causes a premature peak in the flow stress. TABLE 2 AZ31B Normal Mg Alloy Tensile yield Ultimate tensile Anisotropy (room temperature) strength, MPa strength, MPa Elongation*, % e_(u)**, % e_(pu)**, % Ratio (R) As-Received # 160 274 13.0 (13.5) 11.9 1.1 3.8 As-Processed 280 308 22.6 (29.0) 8.4 14.2 6.0 As-Processed + 218 271 24.4 (32.3) 13.4 11.1 5.0 Annealed at 250° C. *Reported elongation is over 12.7 mm gauge length. A shorter gauge length of 5.0 mm gives a higher value of elongation shown in the parenthesis. **e_(u) and e_(pu) refer to uniform strain and post-uniform strain, respectively. # For the as-received material, mechanical test data are from interior region of plate (fine grain region)

Table 2 shows that the fine grain as-processed alloy has improved mechanical properties such as higher tensile yield strength and higher post-uniform elongation, and higher (R) value. Annealing increases tensile elongation values further. When examined for microstructural changes, no twinning was observed in the processed material. Further, the as-received alloy displayed a rough surface similar to “orange peel” white effect, the fine grain processed alloy exhibited a smooth surface after the test. In addition, the degree of necking is found more gradual in the as-processed alloy.

As noted above, grain refinement increases yield strength and reduces strain hardening rate in the fine grain processed alloy in comparison to coarse grain as-received alloy. The high strain rate sensitivity, m, found in the fine grain condition has no connection with texture, but rather is related to the many grain boundaries present in the structure, and it is well-known that a higher m promotes increased elongation by delaying the tendency for strain localization. In the fine grain alloy no twinning was seen, therefore it is believed that dislocation process primarily accommodates plastic deformation in the fine grain alloy. Yield strength for the fine grain alloy is higher due to its finer structure (i.e., smaller spacing between barriers to dislocation) and large fraction of grain (or subgrain) boundaries. More grain boundaries assist in the dynamic recovery process. This, combined with a tendency for shearing or sliding along grain boundaries, increases strain rate sensitivity in the fine grain alloy.

It appears that there is a unique relationship between m and post-uniform elongation, showing that a high value of sensitivity m significantly enhances post-uniform elongation. This relationship appears independent of the alloy system, twinning effect, and texture effect. AZ31B magnesium generally fits this trend, however annealed data shows somewhat higher post-uniform elongation, possibly because of its higher strain hardening capacity.

The somewhat stronger basal texture in the processed alloy may not fully explain the large difference in R-value between these two different grain size conditions. Annealing the as-processed alloy, which appears to not change texture, increases grain size and decreases R-value. It is found that fine grain processing has contributed to changes in R-value, but a reduced value of R did not decrease tensile elongation. Thus annealing effect primarily reduces internal stresses. In the fine grain processed material, the presence of a larger fraction of grain boundary area may favor the activation of non-basal slip for compatibility reasons. Cross slip of prismatic or pyramidal <a> dislocations can promote high R-value in basal textured sheet for in-lane tensile deformation, while extensive <c+a> slip may not increase because a decrease in the R-value, particularly if shearing along many grain boundaries occurs. Thus, non-basal <a> slip may be favored in the fine grain processed alloy for in-plane tension.

In the fine grained alloy, the yield strength for in-plan tension is found to be greater than that for compression even though twinning is inhibited. For metal with limited number of slip systems such as Mg, grain boundary regions experience a greater degree of strain incompatibility and more complex loading than in cubic metals. In ultra-fine grain Mg, where many grain boundaries lead to many locations for incompatibility, changes in local stress-state and stress-concentration can be a significant contributor to the deformation mechanics of polycrystal.

As a further example, an Mg-9Al alloy (AZ91D) sheet bar measuring 100×150×3 mm was semi-solid metal injection molded in a commercial 280 ton Thixomolding® machine at Thixomat, Inc. (Ann Arbor, Mich.). This sheet bar was pressed at 190° C. between opposing sine-wave dies having a corrugated surface pattern through 4 cycles, turning the sheet 900 between cycles. The sheet was press flattened after the 4^(th) pressing cycle. The total reduction of thickness was from 3 mm to 0.8 mm, i.e. 73%. The resultant tensile strengths are compared to commercial AZ31 (Mg-3Al) sheet in Table 3. TABLE 3 Material 0.2% YS, MPa UTS, MPA AZ91D, as injection molded 150 220 sheet bar AZ91D, SWP 4 Cycles 260 300 AZ31, commercial sheet* 150 255 *ASM Handbook

As seen from Table 3, yield strength was increased by 73% compared to the original sheet bar and the commercial AZ31. Ultimate tensile strength was respectively increased 36% and 18%.

This resultant SWP sheet was then annealed at 150 or 250° C. Hardness of the fine grained material derived from the original liquid phase in the as-semi-solid metal injection molded, SWP, SWP+rolled/annealed state were measured and the results are presented in Table 4. TABLE 4 Material Microhardness, MPa AZ91D, as SSMI molded, 5 μm grain size 772 AZ91D, SWP 932 AZ91D, SWP + Annealed @ 150° C. 958 AZ91D, SWP + Annealed @ 250° C. 858 AZ31, commercial sheet, 10 μm grain size 600 AZ31, commercial sheet, 1 μm grain size 720

The fine grained original liquid region of the as semi-solid metal injection molded sheet bar had a 772 MPa hardness, which was increased to 932 MPa by SWP. Annealing at 150° C. increased the hardness further to 958 MPa. Compared to previous data from AZ31, as presented in the graph of FIG. 7, the SWP material from AZ91 D was harder than equivalent grain size AZ31. Part of this hardness increment over AZ31 is believed attributable to nano-size β phase in the Al rich AZ91 D alloy. Microstructures confirmed that the coarse β phase of the starting material had been sub-divided and reprecipitated as nano-particles, some at grain boundaries.

The feasibility of SPF of the SWP sheet has also been demonstrated by the inventors. As FIG. 8 demonstrates, the depth of cup produced via a bulge test of the SWP AZ91D is deeper than that of a sheet (of corresponding thickness) of the starting material formed by Thixomolding (semi-solid metal injection molding process of Thixomat, Inc., Ann Arbor, Mich.) only. In fact, the depth is much greater than that formed in commercial 10-20 μm AZ31 sheet.

Although corrosion was not specifically tested, the 9% Al level of SWP AZ91D should be quite superior to commercial AZ31 (3% Al) in resisting exposure to road or other aqueous environments.

While illustrated above with Mg alloy, other alloys, capable of being processed into a precursor work piece having an initial fine grain structure, are believed to be suitable to the present invention and include, without limitation, Al, Zn, Ni, Cu, α/β Ti, steels, duplex α/γ stainless steels, α/γ steels, γ/martensite Maraging steels and metal/ceramic particle composites.

Potential markets for products manufactured by the present invention are envisioned in the automotive and aerospace fields, among others, where weight savings can be gained by replacing steel and aluminum with magnesium. Complex 3-D net-shapes can be SPF to greatly reduce the number of sub-assemblies and the costs of multiple fabrication and assembly. High tensile strength and high toughness will be attained by sub-micron grain sizes, second phase nanocrystals and by the selection of ductile alloys. The unique microstructure so attained will greatly reduce texture and its usual barrier to formability.

Automobile companies are predicting very significant increases in Mg tonnage for automotive vehicles, as much growth as from 5 Kg/car up to 200 Kg/car. There is a need to enable the United States automotive industry to lead this sea change in light-weighting. Additional markets should further open in the aerospace, defense and other industries.

SWP of suitable alloys should reduce the cost of making thin sheet material by eliminating multiple stages of rolling and annealing. Deformation by SWP changes the grain boundary character and increases the ability to be formed by warm forming or by superplastic deformation. If sinusoidal deformation is carried out immediately following injection molding, the sensible heat in the molded blank can be utilized. Following immediate rolling or pressing of the sheet bar, it can be formed by SPF into complex part shapes. Such forming can be accomplished at 200° C. Thus, the entire component fabrication technology can be set into a continuous operation without storage of coils of sheets, considerable coil annealing, coiling and uncoiling operations. The removal of all of the steps involved with coiling and cranes handling transport of coils would minimize investment in plants. A leaner manufacturing process for parts would emerge.

It is envisioned that SWP can be accomplished by integrating injection molding machines for metal with conventional pressing and rolling equipment and should be feasible on process equipment already used in the aerospace and automotive industries. Deep drawing also can be practiced on conventional presses.

As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementations of the principles of this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from spirit of this invention, as defined in the following claims. 

1. A method of forming a sheet material having a refined grained structure, the method comprising the steps of: providing a metal material; molding and rapidly solidifying the metal alloy to form a fine grain precursor; and imparting plastic deformation to the fine grain precursor by a combination of alternating tensile strain and compressive strain to form an ultra fine grain structured sheet form.
 2. The method of claim 1 wherein the step of molding and solidifying develops a mutliphased microstructure in the fine grained precursor.
 3. The method of claim 2 wherein the multiphased microstructure includes pinning particles that minimize grain growth.
 4. The method of claim 1 wherein the step of imparting plastic deformation includes the step of storing dislocations in the microstructure.
 5. The method of claim 1 wherein the step of imparting plastic deformation includes the step of causing the formation of new grain boundaries having high misorientation suitable for warm forming or superplastic forming.
 6. The method of claim 1 wherein the molding step and the imparting plastic deformation step are performed in an integrated apparatus.
 7. The method of claim 1 wherein the molding step and the imparting plastic deformation step are performed by separate machines.
 8. The method of claim 1 wherein the molding step includes semisolid metal injection molding of the metal material.
 9. The method of claim 1 wherein the molding step includes one of extruding of the metal material and twin roll casting of the metal material.
 10. The method of claim 1 wherein the imparting plastic deformation step includes corrugating the precursor in a first direction and subsequently corrugating the precursor in a second direction.
 11. The method of claim 10 wherein the second direction is orthogonal to the first direction.
 12. The method of claim 10 wherein the second direction is aligned with the first direction.
 13. The method of claim 10 wherein the imparting plastic deformation step further includes the step of flattening the precursor.
 14. The method of claim 13 wherein the flattening step is performed after at least one of the steps of corrugating the precursor in the first direction and the second direction.
 15. The method of claim 14 wherein the imparting plastic deformation step further includes the step of corrugating in a third direction and a fourth direction.
 16. The method of claim 15 further wherein a second flattening step is performed after at least one of the third and fourth corrugating steps.
 17. The method of claim 1 further comprising, after the step of imparting plastic deformation, the step of net shaping the nano-sized grain structure sheet.
 18. The method of claim 17 further comprising the step of heat treating the net shaped part to impart creep resistance to the net shaped part.
 19. The method of claim 17 wherein the step of net shaping includes one of stamping, drawing, deep drawing and superplastic forming.
 20. The method of claim 17 wherein the step of net shaping forms an automotive component.
 21. An apparatus for performing the method of claim
 1. 22. An article formed by the method of claim
 1. 23. The method of claim 1 further comprising the step of providing the sheet form with a thickness being less than that of the precursor.
 24. The method of claim 1 wherein the metal material is a metal alloy.
 25. The method of claim 1 wherein the metal material is a magnesium alloy.
 26. The method of claim 1 wherein the metal material is one selected from the group of aluminum alloy, zinc alloy, nickel alloys, copper alloy, α/β titanium alloy, steels, duplex α/γ stainless steels, α/γ steels, γ/martensite Maraging steels and metal/ceramic particle composites.
 27. The method of claim 1 wherein the step of imparting plastic deformation includes die pressing of the fine grain precursor.
 28. The method of claim 1 wherein the step of imparting plastic deformation includes rolling the fine grain precursor.
 29. The method of claim 1 wherein sheet form is provided having a grain structure of less than about 2 micrometers.
 30. The method of claim 1 wherein the sheet form is provided having a grain structure of less than about 1 micrometer.
 31. The method of claim 1 where the step of imparting plastic deformation is performed while the precursor is heated above ambient.
 32. The method of claim 1 wherein the step of imparting plastic deformation imparts tensile strain and compressive strain in a strain direction.
 33. The method of claim 32 wherein the step of imparting plastic deformation is performed by passing the precursor in a first direction between at least one pair of deforming members having corrugated surfaces, and wherein the first direction is orthogonal to the strain direction.
 34. The method of claim 33 wherein the compressive strain is imparted at least in part by flattening the work piece while constraining lengthening of the work piece in the strain direction.
 35. The method of claim 34 whereby constraining lengthening of the work piece in the strain direction is done so as to achieve one of decreasing, increasing or preserving the thickness of the precursor in the sheet form.
 36. An apparatus for refining grain structure and producing ultra-fine grained metal material sheets, the apparatus comprising: a receptacle having an inlet, a discharge outlet remote from the inlet, and a chamber defined between the inlet and the discharge outlet; a feeder coupled with the inlet, the feeder configured to introduce an metal material into the chamber via the inlet; a heating device for transferring heat to the metal material located within the chamber such that the metal material is at a temperature above its solidus temperature; discharge means for discharging the metal material from the receptacle through the discharge outlet; forming means for forming and rapidly solidifying the discharged metal material into a fine grained precursor; plastic deformation means for imparting tensile and compressive strain into the precursor article, the plastic deformation means deforming the precursor article into a corrugated work piece and including a pair of opposing forming members having protrusions formed on a surface thereof, the protrusions of one forming member being offset from the protrusions of the opposing forming member; the plastic deformation means further including flattening means for flattening the corrugated work piece into a sheet form of the metal material having an ultra-fine grain size.
 37. The apparatus of claim 36 wherein the opposed forming members are dies.
 38. The apparatus of claim 36 wherein the opposed forming members are rolls.
 39. The apparatus of claim 36 wherein the plastic deformation means includes means for imparting tensile strain and compressive strain into the precursor article.
 40. The apparatus of claim 39 wherein the plastic deformation means includes means for imparting first corrugations oriented in a first direction into the precursor article and subsequently imparting second corrugations oriented in a second direction.
 41. The apparatus of claim 40 wherein the second direction is orthogonal to the first direction.
 42. The apparatus of claim 40 wherein the second direction is aligned with the first direction.
 43. The apparatus of claim 40 wherein the plastic deformation means is configured to impart third corrugations and fourth corrugations into the work piece.
 44. The apparatus of claim 43 wherein the third and fourth corrugations are respectively oriented in the direction of the first and second corrugations.
 45. The apparatus of claim 44 wherein the third and fourth corrugations are out of phase with the first and second corrugations.
 46. The apparatus of claim 45 wherein the third and fourth corrugations are respectively 180 degrees out of phase with the first and second corrugations.
 47. The apparatus of claim 36 further comprising net shaping means for shaping the sheet form of the metal material into a net-shaped article.
 48. The apparatus of claim 47 wherein the shaping means is one of a drawing press and a superplastic forming machine.
 49. The apparatus of claim 36 wherein the receptacle, feeder, heating means, discharge means and forming means are part of an injection molding machine.
 50. The apparatus of claim 36 wherein the receptacle, feeder, heating means, discharge means and forming means are part of a semi-solid metal injection molding machine.
 51. The apparatus of claim 36 wherein the plastic deformation means imparts tensile and compressive strain in a strain direction that is orthogonal to a direction through which the work piece is passed through the plastic deformation means.
 52. The apparatus of claim 51 wherein the flattening means imparts, at least in part, compressive strain to the work piece.
 53. The apparatus of claim 52 wherein the flattening means includes features to control lengthening of the workpiece in the strain direction whereby the thickness of the sheet form may be controlled so as to be increased, decreased or the same as the thickness of the precursor.
 54. A method of forming a sheet material having a refined grained structure, the method comprising the steps of: providing a metal material; molding and rapidly solidifying the metal alloy to form a fine grain precursor defining a line length; initially increasing the line length of the precursor to form a work piece; after the step of initially increasing the line length, decreasing the line length of the work piece; after the step of decreasing the line length, then increasing the line length of the work piece; and flattening the work piece to form an ultra fine grain structured sheet form.
 55. The method of claim 54 further wherein the step of flattening the work piece is performed before the decreasing and increasing step and then again after the decreasing and increasing step.
 56. The method of claim 54 wherein the step of flattening the work piece is performed after the decreasing step and the subsequent increasing step.
 57. The method of claim 54 wherein the initially increasing step introduces strain into the work piece in a first direction.
 58. The method of claim 54 wherein the decreasing and increasing step introduces strain into the work piece in a second direction.
 59. The method of claim 58 wherein the second direction is orthogonal to the first direction.
 60. The method of claim 58 wherein the second direction is generally aligned with the first direction. 